The invention relates to a method and apparatus for managing the penetration of high energy, high velocity plasma into a combustion chamber of an internal combustion engine or a continuous combustion system.
With the need both to conserve fuel and to protect the environment, new methods are sought to increase the efficiency of conventional internal combustion engines or continuous combustion systems. One method is to operate the engine at a much leaner fuel-to-air mixture. This will reduce the fuel requirements while also lowering the amounts of pollutants emitted into the air. Although using leaner mixtures seems an obvious solution, various problems arise in developing lean-burning engines. One problem is that leaner mixtures do not reliably ignite with conventional electrical spark mechanisms. In order to ignite the leaner mixtures, a much hotter electrical energy source is required. Thus, researchers have turned to high energy plasma jet ignitors as a means for providing the necessary higher ignition temperature for lean-burn engines.
In addition, ignition does not guarantee effective combustion of the fuel-air mixture. Lean mixtures have a much lower unstretched laminar flame speed (a thermochemical property) than the stoichiometric to slightly rich mixtures commonly used. This slow burning results in decreased thermal efficiency of the engine and a consequent increase in fuel consumption. Further, lean mixtures have a low energy release rate per unit volume of combustible mixture. If the ignition source is located adjacent to the relatively cool combustion chamber walls, as with a conventional spark plug, the rate of heat loss to the walls can be greater than the energy liberation rate due to combustion, thus leading to flame quench, incomplete combustion, increased fuel consumption, and increased hydrocarbon emissions. Further, as the mixture becomes increasingly lean, misfire and partial burn limits are encountered. Eventually the lean operating limit is encountered. The net result is that, as the mixture becomes progressively leaner, the hydrocarbon emissions begin to increase rather than decrease as expected.
The "partial burn limit" is a result of flame stretch extinguishing the flame during early flame development. The unconsumed fuel, in the region of the combustion chamber through which the flame has not passed, appears in the exhaust as unburned hydrocarbons. The "lean operating limit" is defined as the mixture for which the coefficient of variation of the indicated mean effective pressure becomes excessive or, in other words, the cyclic variability is sufficiently severe that the engine operates erratically. This is most important at idle. This cyclic variability is due to three factors: misfire on some cycles, partial burn on other cycles, and on the remaining cycles, variation of the rate of combustion during the early stages of combustion. The variation of the initial rate of combustion has been shown to be due to differences in the direction of migration of the initial spark kernel, which is being pushed around by relatively larger turbulent eddies. Thus, it has been argued that cyclic variations cannot be established once the flame kernel is larger than a critical size.
One solution to these problems is to ignite the mixture on a larger (global) scale instead of at a point. This reduces the distance the flame must propagate and minimizes flame quench. Further, the ignitor should induce turbulence in the combustion chamber. This increases the burning rate. One method proposed for achieving global ignition and inducing turbulence is the use of plasma jet ignitors for leanburn engines.
Many types of plasma jet ignitors have been proposed. The earliest was originally developed in Russia and is disclosed in U.S. Pat. No. 4,041,922. This type of plasma jet ignitor is also known as a torch cell or torch ignitor and uses a prechamber separated from the main combustion chamber by an orifice. An essentially conventional spark plug is located in the prechamber. When the spark jumps the spark plug gap, a very small amount of plasma is formed in the arc. This provides the energy to ignite the combustible mixture in the prechamber. The orifice serves to pressurize the reacting mixture until a jet of reactive species issues from the prechamber and into the main combustion chamber, thereby serving to ignite the lean mixture in the main combustion chamber. Because a conventional spark plug is used in the prechamber, an essentially stoichiometric or rich mixture must be present in the prechamber but the mixture in the main chamber can be sufficiently lean that the overall mixture is lean. Thus, this device achieves an ignition jet solely through thermal expansion of the combustible mixture in the prechamber and the jet mixture is relatively cool. The jet velocity is much less than the velocity of sound and the jet temperature and velocity decline as the jet expands.
Another type of plasma jet ignitor is described in U.S. Pat. No. 3,911,307. It is similar to a conventional spark plug but embodies a recessed center electrode and an orifice cap over the cavity formed by recessing the center electrode. The orifice cap serves as the ground electrode. When an arc jumps between the center electrode and the ground electrode, the gases within the arc become ionized. The remaining gases in the cavity are thermally heated by heat transfer from the ionized gases. The orifice serves to pressurize these gases until a jet of reactive species issues from the cavity and into the combustion chamber, thereby serving to ignite the lean mixture in the combustion chamber. Thus, this device achieves an ignition jet solely through thermal expansion of the gases in the cavity and the jet mixture is relatively cool. The jet velocity is limited to the velocity of sound. Also, the jet temperature and velocity decline as the jet expands.
Another type of plasma jet ignitor is described in U.S. Pat. No. 4,122,816. This plasma jet ignitor also has a cavity and an orifice which separates the cavity from the combustion chamber. The orifice is an annulus surrounding the center electrode. The ground electrode surrounds the external portion of the orifice. When an arc jumps the gap across the orifice, the gases within the arc become ionized. The gases within the cavity are thermally heated by heat transfer from the ionized gases. The orifice serves to pressurize the gases within the cavity and the resulting thermal expansion forces a jet out of the cavity. If a combustible mixture is contained within the cavity, the heat transfer will ignite this mixture, providing additional thermal expansion. Additionally, since the inner and outer electrodes are essentially parallel for a short distance, an electromagnetic force is developed which accelerates the plasma away out of the orifice. Thus, this plasma jet ignitor develops a jet both through thermal expansion and through electromagnetic acceleration. However, this device has an essentially continuously increasing plasma surface area and a continuously decreasing magnetic field strength. This results in a continuously decreasing electromagnetic force available to accelerate the plasma. Further, because the electrodes are exposed to each other for only a very short length, the plasma is weakly accelerated from zero velocity for only a very short period of time. Thus, the resulting electromagnetic pressure has been shown to be negligible, and the device achieves an ignition jet primarily through thermal expansion of the gases in the cavity and the jet mixture is relatively cool. The jet velocity is much less than the velocity of sound and the jet temperature and velocity decline as the jet expands.
The plasma jet ignitors discussed above attempt to solve the problems encountered in developing lean-burn engines. U.S. Pat. Nos. 4,203,393 and 4,398,526 recognize that ignition system difficulties may be important for other engine applications. These patents refer to use of plasma jet ignitors similar to that disclosed in U.S. Pat. No. 4,041,922 as applied to direct injection stratified charge spark ignition engines and spark assisted diesels, respectively. In fact, ignition system problems may affect or limit the design of most types of engines and continuous combustion systems.
For conventional spark ignition engines, an increased rate of combustion is advantageous for increasing thermal efficiency, fuel economy, and performance. A high velocity jet of reactive species would serve to decrease the duration of combustion. Cyclic variability, knock tendency, and fuel sensitivity should also decrease. As a second example, achieving cold start of diesel engines is a significant ignition related problem. The ability to produce a high energy plasma jet that can penetrate across the combustion chamber without the need to have a combustible mixture in a cavity or spark initiation gap would result in the ability to force ignition rather than relying on autoignition. Such a device could be used to replace glow plugs in indirect injection diesels or as a cold starting aid for direct injection diesels, either in-cylinder or as the ignitor for a fuelburning manifold heater.
As another example, the assurance of achieving high altitude relight of aircraft gas turbines is an ignition problem. The ability to produce a high energy plasma jet that can penetrate across the combustion chamber without the need to have a combustible mixture in a cavity or spark initiation gap would be very advantageous for this application.
As another example, 2-stroke spark ignition engines normally misfire on as many as 30% of the engine's cycles at part load, obviously another ignition related problem. As yet another example, methanol fueled spark ignition engines need a high heat range spark plug for cold starting but a lower heat range spark plug for normal operation to avoid preignition off the hot spark plug. Similarly, dual-fuel spark ignition engines (spark ignition engines designed to be operated on gasoline, alcohol, or any blend such as M85-85% methanol and 15% gasoline) must use a spark plug that is three heat ranges lower when using methanol than when using gasoline, to avoid preignition off the hot spark plug. However, lower heat range spark plugs exhibit severe cold fouling problems when gasoline is being used. What is needed is a "cold" plug that resists fouling but delivers sufficient energy to ignite the mixture under adverse conditions.
There are various ways of powering a plasma jet ignitor, the most popular being either a SCR capacitor discharger or a thyrathron tube, each having a power supply booster circuit. The boosted injector power source can deliver up to 10 Joules per pulse to the injector, which is then selectively channeled to the appropriate plasma jet ignitors.
The design of both the power source and the plasma jet ignitor geometry also demand particular attention. In order to achieve optimal penetration of the jet, it is important that the power source optimize the energy delivered to the injectors while maintaining optimal pulse duration, and also that the point of injection be optimally focused near the center of the combustion chamber. Conventional plasma jet ignitors are limited to only 2 to 3 cm penetration which has proved unsuitable to allow lean-burn engines to achieve sufficiently lean operation for improved fuel economy and decreased emissions.